1. Field of the Invention
The present invention relates to a pad for an air bag device located in a steering wheel of an automobile, and more particularly to the material to be used for the pad.
2. Description of the Related Arts
Recently more and more automobiles have been provided with air bags to protect the driver and occupants of the automobile from the effects of a collision. The air bag may be located in the steering wheel, the front panel under the dash board, the back part of the front sheet, and the like.
In a related development, U.S. Pat. No. 4,148,508 (Shiratori et al.) disclose a gas bag and gas generator located in a receiving case at the center portion of the steering wheel. The receiving case included a lid surface on the outside front of the steering wheel. This lid surface has fragile portions which rupture when the gas generator operates to generate large quanties of gas in the gas bag. The receiving portion including the lid surface is made of a thermoplastic elastomer which has sufficient strength so that, when the gas generator operates, the receiving portion fractures only at the fragile portions on the lid surface and nowhere else. The outside of the lid surface is covered by a pad which is made of a material retaining flexbility and being prone to rupture.
U.S. Pat. No. 5,002,306 (Hiramitsu et al.) discloses a similar inflatable air bag device installed in a steering wheel with a pad covering the device. The pad is made of soft synthetic resin such as urethane, polyvinyl chloride, etc. A break-open area defined by a thinned breaking portion in an H-like shape is formed on the upper wall of the pad.
U.S. Pat. No. 4,852,907 (Shiraki et al.) discloses a similar inflatable air bag device installed in a steering wheel with a pad covering the device. The pad is designed so that an insert can be molded integrally with the pad. The insert comprises a rectangular tubular base portion made from a synthetic resin and a plurality of flexible nets.
As shown in FIGS. 1 and 2, an air bag 21 is located in the central portion of a steering wheel 15. A pad 10 for the air bag device is provided around the upper periphery of the air bag 21 at the driver's side.
The pad 10 is provided with a schematic H shaped slit 11 from where the pad 10 ruptures at the inflation of the air bag 21. Both upper and lower surfaces of the pad 10 are provided with the slit 11 and a thin wall portion 110 is provided between the upper surface slit 11 and the lower surface slit 11. (See FIG. 3.)
As shown in FIG. 1, a switch 31 is turned on at an electrical circuit to heat a heater 33, resulting in a large amount of gas 29 generated through a gas foaming agent 25. The gas 29 is quickly supplied into the air bag 21 to inflate the air bag rapidly. Then the air bag 21 bursts through the thin wall portion 110 and expands in front of the driver to protect the driver's face.
During the above inflation of the air bag, it is most efficient to have the pad rupture along a rupture line 41 at the thin wall portion 110 between upper and lower slits 11 as shown in FIG. 3.
However, as shown in FIG. 4, the pad 10 ruptures in an abnormal and undesirable way along a line 42 out of the thin wall portion 110.
Accordingly various examinations were made in order to produce a normal rupture as shown in FIG. 3, i.e., the rupture occurring along with or close to the rupture line 41 at the thin wall portion, leading to the present invention.